In contemporary radio communication systems, including cellular systems, such as e.g., 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, the transmission format used is adapted to the existing radio environment in order to maximize the spectral efficiency. Such adaptation encompasses the modulation level and channel coding rate. The adaption may also include adjustment of Multiple-Input and Multiple-Output (MIMO) related formats, e.g., adaption of used precoder matrix, rank of data transmission, or the like.
In order for a node in the radio communication system, such as a base station/eNode-B (eNB), to be able to perform adaptation for its downlink transmissions, usually some feedback relating to the downlink channel information is transmitted from the mobile/User Equipment (UE). In LTE systems, implicit measures are used for this feedback, e.g., the UE may report a Channel Quality Indicator (CQI) value to indicate suitable modulation and coding format to be used, as well as Precoding Matrix Indicators (PMIs) and Channel Rank Indicator (RI) on the uplink channel.
In 3GPP LTE, the transmission of CQI/PMI/RI reports can be either periodic or aperiodic. Periodic reports are transmitted in the Physical Uplink Control Channel (PUCCH). These periodic reports are transmitted on resources being configured by the eNB, wherein the eNB also configures the periodicity of the reports.
The aperiodic reports can referred to as triggered reports, or scheduled reports, and can contain more information than the periodic reports. Such information can include e.g., the CQI, PMI and RI, and is scheduled by the eNB via an uplink grant (UL grant) in the Physical Downlink Control Channel (PDCCH). The information is transmitted in the Physical Uplink Shared Channel (PUSCH). Also, since simultaneous transmission of PUCCH and PUSCH is not allowed in LTE, the periodic report is multiplexed into the PUSCH, if there is a collision with UL data transmission.
In 3GPP LTE-Advanced systems, carrier aggregation can be used, which implies that a UE simultaneously can receive and transmit on more than one downlink (DL) and uplink (UL) component carrier, respectively. If one UE is scheduled to transmit data on multiple DL component carriers, there will be one PDCCH corresponding to each scheduled downlink component carrier. Correspondingly, if one UE is scheduled to transmit data on multiple UL component carriers, there will be one PDCCH corresponding to each scheduled uplink component carrier.
When using carrier aggregation in LTE-Advanced, at most five downlink component carriers, and/or at most five uplink component carriers can be aggregated. The carrier aggregation can be asymmetric, i.e., a UE may have more downlink component carriers than uplink component carriers.
An LTE-Advanced UE can utilize a number of component carriers for carrier aggregation, where these carriers are configured by the Radio Resource Control (RRC) signalling by the eNB, i.e., the carrier aggregation used can be UE-specific. Also, the eNB may among the number of configured downlink component carriers deactivate some of these downlink carriers. For such a deactivated downlink component carrier, the UE is neither monitoring the PDCCH, nor receiving the Physical Downlink Shared Channel (PDSCH). The UE does also not report any CQI/PMI/RI for this deactivated downlink component carrier. Carrier activation/deactivation is here facilitated by Media Access Control (MAC) signalling, which can be performed faster than the carrier configuration/reconfiguration handled by RRC signalling.
It is also considered in prior art to define a PDCCH monitoring set, i.e., a subset of the active component carriers for which the UE is monitoring the PDCCH. The use of such a monitoring set may ease the processing burden of the UE, which otherwise blindly has to search for the PDCCH in its search spaces. Generally, a UE in the system searches for the PDCCH in two search spaces; a common search space and a UE-specific search space. These search spaces are comprised by a number of control channel elements defining positions in the time-frequency domain, on which the PDCCH can be transmitted. The location of the PDCCH is not known by the UE, and the UE therefore blindly has to search several possible candidate positions in such search spaces.
Furthermore, LTE-Advanced supports cross-carrier scheduling, i.e., the PDCCH of a downlink component carrier can assign PDSCH or PUSCH transmission resources in one of multiple downlink component carriers and uplink component carriers, respectively, by utilizing the carrier indicator field (CIF) in the PDCCH. In LTE-Advanced, the configuration of the presence or non-presence of CIF bits in the PDCCH is semi-static and UE specific. Thus, the configuration of the CIF bits is not system-specific or cell-specific. Also, these CIF bits do not have to be configured for all downlink component carriers.
When used, the CIF bits have a length being equal to 3 bits. The CIF bits define a number of CIF states. Each CIF state is here associated with one of the aggregated component carriers. The CIF in the DL scheduling PDCCH indicates that one PDSCH is scheduled on the active downlink component carrier corresponding to the state indicated by the CIF. However, the CIF in the UL grant PDCCH indicates that one PUSCH will be scheduled on the active uplink component carrier corresponding to the state indicated by the CIF.
A number of different Downlink Control Information (DCI) formats can be transmitted on the PDCCH for different purposes. The PUSCH, which can include UL data and/or CQI/PMI/RI reports, is scheduled through DCI Format 0.
For cross-carrier scheduling, explicit CIF indications are included at least for DCI formats 0, 1, 1A, 1B, 1D, 2, 2A, 2B in the UE-specific search space. CIF bits are not included in Format 0 scheduled in the common search space. However, CIF bits may be included in new LTE-Advanced formats, e.g., those that are needed for supporting UL MIMO or enhanced DL MIMO.
In 3GPP LTE, a UE searches for DCI Format 0 and 1A simultaneously. In order to simplify the decoding effort, the payload sizes of Format 0 and 1A are matched to be of the same length. So called padding bits are appended to the information fields in these formats to assure that the number of bits are the same. That is, if the number of information bits in Format 0 is smaller than in Format 1A, zeros are padded until the total number of bits is the same as the number of information bits of Format 1A, or vice versa.
The periodic CQI/PMI/RI reports are in LTE-Advanced transmitted on a single UE-specific uplink component carrier. This specific uplink component carrier may be referred to as primary component carrier (PCC), or anchor carrier. Other component carriers, i.e., non-PCCs, may be referred to as secondary component carriers (SCCs). Generally, it can be assumed that several UEs in a cell are configured for carrier aggregation. However, most of the time, usage of one component carrier might be sufficient for each UE. Thus, most of the time only a few UEs in a cell may need to use multiple active carriers for their PDSCH/PUSCH transmission. That is, it can be expected that, most of the time, a UE confines its PDSCH/PUSCH transmissions to the PCC.
In LTE, an aperiodic CQI/PMI/RI report is enabled by a CQI request bit in Format 0 of the UL grant PDCCH. It is possible to concurrently send the aperiodic CQI/PMI/RI report with other data and if the UL grant also schedules UL data, the CQI/PMI/RI report is punctured into the PUSCH. The information being present in the report depends on the transmission mode and the CQI reporting mode.
LTE does not support carrier aggregation, and consequently the report contains CQI/PMI/RI information relating to the only DL carrier used for the UE. Also, the report is transmitted on the PUSCH of the only UL carrier used for the UE (for FDD). In case the aperiodic report would collide with the transmission of a periodic report, the periodic report is dropped. Aperiodic CQI/PMI/RI report can also be requested through a random access response grant, which contains one CQI request bit.
However, since carrier aggregation utilizes more than one component carrier when carrier aggregation is configured for a UE, e.g., in an LTE-Advanced system, aperiodic feedback, such as channel quality reports, for multiple active downlink component carriers cannot be performed by utilizing the known prior art solutions. Also, the prior art solutions for transmission of such aperiodic feedback on the PUSCH do not work for systems using carrier aggregation, for example if the number of downlink component carriers is greater than the number of uplink component carriers. Thus, if directly applying prior art solutions on such a carrier aggregation system, the channel quality reports would not reliably reach the network node requesting the reports.
Also, aperiodic channel quality reports need to be scheduled, which requires signalling. If prior art scheduling solutions would be directly applied to a carrier aggregation system, it would result in a substantive amount of overhead being associated with this scheduling.
Thus, the prior art fails to provide a low overhead solution for aperiodic downlink channel quality reporting for a system utilizing carrier aggregation.